Pretty simple really. A basic mixer allows you to take multiple inputs and run them through the same output. So, let's say you have a 4-track mixer. Well, you can plug in 4 guitars and have them all come out of the same set of speakers, or in your case, into the computer line in via an adaptor.

That's a basic mixer, fairly straightforward.

Usually mixers have some form of rudimentary Equalizer, and they also provide volume control over each input as well as an independent headphone output with volume control.

Also, this thread belongs in recording gear and equipment, and I am moving it now.